Simple fix, just figured it out myself.
Similar to your issues I am sure.
So it turns out I also have an amazon.co.uk account.
I downloaded the Alexa app from the US iTunes Store back in the days that was the only geo you could download it from. and I assume this is hardcoded to Amazon.com. Also if you google Alexa Web App you will get a login page
https://www.amazon.com/ap/signin?... note this is also hardcoded to amazon.com
So here is the weird bit, even though your account is amazon.co.uk the amazon.com iOS and web app just lets you log in? so you think all good? but no this creates a completely separate profile, you can see this if you delete .com and replace it with .co.uk in the url, then log in. Suddenly all my devices "show" and "spot" are online and I can see my news briefings and other settings are all back to default settings.
Not sure why the web app and iOS app when you log in does not error or simply redirect you to the correct login page?
Any how, all devices now online, absolutely nothing to do with wifi connectivity, I can understand why there are so many people confused by this.
I'll download the UK iTunes Alexa app and see if that solves my offline issues on the iOS app.
Thank you Tronski for pointing me in the right direction.
All the best,
Simon